In what condition could a HDD be wiped out all of data includes partitioning and become wholely unallocated

Very sad , yesterday I had an accident , I realized that my HDD 80GB Seagate Barracuda could not boot anymore . It said " Invalid system disk " .
Very quickly , I used some boot disk to check and took that HDD to another PC , I recognized that what i could see was just a 74.5GB hard disk unallocated . Shock !
I swear the previous day I was chatting and sufing web , no things ever happened . It just like .. I dont understand ?
I have used Hiren boot disc , trying to recover but with no luck ., tried with testdisk , active partition recovery .. , I have spent days hoping to get it back but failed . I even cannot recover just some archieve files , it seems the hard disc was wiped so clean .

The drive does spin well and act as if no thing ever happened to it .
When I plugged the drive into a different system , disc management showed that there is 1 unallocated 74.5 GB , wholely unallocated and it does not have any format .

Actually , and I regret that I have tried Testdrive and agreed it to recover last 2 partition ( my drive had totally 1 primary and 4 logical ) . Actually it didnt work also ( because windows saied it could not access those partition , anyways , data I treasure is in partition 2 and 3 ) . In despair , I thought that may be I could do a quick format to let recover software " see the unallocated partition " , then use recover software to get data back . I did , and failed . No thing ever back . My mistake again , I know I have reduced the chance of recovering data . Blameful !

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